This invention relates to thermal transfer printers, dyesheets therefor and methods of operation thereof.
According to one aspect of the invention there is provided a thermal transfer printer including detector means for detecting a light absorption characteristic of a thermal transfer dyesheet inserted in the printer, comparison means for comparing the detected light absorption characteristic with an acceptable light absorption characteristic and rejection means for preventing use or further use of the dyesheet in the printer if the detected light absorption characteristic fails to conform to the acceptable light absorption characteristic.
The detector means may be operative to detect the light absorption characteristic of one colour only of a multi-colour dyesheet, but to improve discrimination the detector means is preferably operative to detect the respective light absorption characteristics of more than one colour, the rejection means then preventing use or further use of the dyesheet if the detected light absorption characteristic of any one colour fails to conform to the acceptable light absorption characteristic for that colour. For each colour detected, the detector means preferably comprises a light source of a frequency appropriate to the colour to be detected and a detector which produces an electrical output signal representative of the attenuation of the light as a result of passage of the light through the colour of the dyesheet. The light source and detector may be on opposite sides of the plane of dyesheet movement through the printer or may be on the same side, the light then being transmitted a first time through the dyesheet, being reflected and then being transmitted a second time through the dyesheet. It is also possible to obtain further discrimination by measuring the light absorption characteristic of a black or overlay panel of the dyesheet.
Preferably, the detected light absorption characteristic is a magnitude of light absorption and the acceptable light absorption characteristic is a range of light absorption values, the rejection means then preventing use or further use of the dyesheet in the printer if the detected light absorption magnitude falls outside the acceptable range. It is convenient to quantify the absorption magnitude by taking the ratio of the detector output with the dye panel in place to the detector output on a clear portion of the dyesheet.
It is also possible for the light absorption characteristic to be the magnitude of optical density, where optical density has its conventional definition of log10 (I0/I), in which I0 is the intensity of the incident light and I is the intensity of the transmitted light.
The rejection means may operate in any one of a number of ways. For example, the rejection means could prevent use or further use of the dyesheet by disabling an essential function of the printer such as dyesheet transport or operation of the print head, or the rejection means could eject the dyesheet from the printer, this being most practicable if the dyesheet is carried in a cassette or cartridge. In each case, the printer could produce an audible signal and/or a visual indication to the user that the dyesheet is not acceptable.
According to another aspect of the invention there is provided a method of determining the acceptability of a thermal transfer dyesheet in a thermal transfer printer, the method comprising determining a light absorption characteristic of the dyesheet, comparing the detected light absorption characteristic with an acceptable light absorption characteristic and preventing use or further use of the dyesheet in the printer if the detected light absorption characteristic fails to conform to the acceptable light absorption characteristic.
The light absorption characteristic may be determined by determining the intensity of light (of a chosen frequency) transmitted by a colour print panel of the dyesheet.
The dyesheet is normally fed from material wound up on a spool and is taken up after use on a second spool. In order to interrogate the successive panels of a dyesheet, it should desirably be wound past the detectors. Three possibilities are:
(i) After the dyesheet has been loaded into the printer, part of the installation procedure (eg closing the lid of the printer) triggers the detection process, which is carried out by winding forwards through a complete sequence, thus wasting one repeat unit of the dyesheet. This may not be of great consequence if there are several hundred repeats on the dyesheet spool. It does, however, mean that only a single check is made at the beginning, and subsequent panels could be out of specification.
(ii) After the dyesheet has been loaded, and at certain other times, the dyesheet is wound forwards to confirm its identity, and then wound back again, so that none is wasted. This would be a relatively slow process because of the need to wind the dyesheet in both directions.
(iii) Printing is carried out as normal, while simultaneously monitoring the light absorption of the dyesheet. If the dyesheet is inappropriate, the print cycle is aborted. This is potentially the simplest method to use, and in the event that the wrong dyesheet is used would limit wasted material to one unit of dyesheet and receiver.
Instead of interrogating colour print panels of the dyesheet, the intensity of light transmitted through a sample colour area on the dyesheet, corresponding to a colour print panel, may be determined in order to derive the light absorption characteristic. Preferably, these sample areas are interrogated by the printer before commencing printing, avoiding any additional winding or rewinding.
According to a yet further aspect of the invention there is provided a thermal transfer dyesheet for use in a thermal transfer printer, the dyesheet comprising colour print panels arranged in series along the length of the dyesheet, the colour print panels being arranged in repeating groups with each group comprising print panels of three different colours, between each group there being three sample colour areas spaced across the width of the dyesheet and corresponding in colour to the three colours of the print panels. The three different colours may be yellow, magenta and cyan, and there may also be a registration mark between each group of colour print panels.